Immune responses in neonates are often characterized by high level production of anti-inflammatory Th2 cytokines. This pattern of cytokine secretion is thought to contribute to the susceptibility of young animals to infection and to the development of Th2-mediated diseases, such as asthma. Thus, vigorous Th2 function is a key defining feature of immunity in early life. Learning how this arises is of central importance for our understanding of the ontogeny of the adaptive immune system. We have recently demonstrated that murine neonates are epigenetically poised for robust Th2 function. Unlike na[unreadable]ve adult CD4+ cells, na[unreadable]ve neonatal CD4+ cells are hypomethylated at CpG residues in key regulatory sites of the Th2 cytokine locus. This hypomethylated state is strongly linked to rapid, high level Th2 cytokine production by neonatal cells. Thus, developmental differences in epigenetic patterns appear to play a central role in the functional disparities between Th cells in early life and adulthood. Importantly, this hypomethylated state is established in fetal life, as early as 14 days of gestation and within the earliest cells to enter the fetal thymus. In addition, the hypomethylated pattern appears to be restricted to T cell precursors and CD4+ cells and, among major immune effector loci, is found only at the Th2 cytokine locus. The overall goal of this proposal is to learn how the unique epigenetic signature at the Th2 locus is regulated, at the cellular and molecular levels, in early life. We will compare the DNA methylation status of pre-thymic progenitor cells in the fetal liver and adult bone marrow. We will also determine whether the hypomethylation is due to progenitor cell intrinsic properties by assessing the methylation status of progeny arising from fetal or adult T cell precursors within a fetal or adult thymic environment. Candidate molecular regulators of the DNA methylation program, identified by previous microarray analyses, will be tested by examining the methylation status in fetal, neonatal, and adult mice deficient in or overexpressing the master transcription factors of the Th2 or Th1 lineage, GATA-3 or T-bet, respectively. Because histone modifications can influence DNA methylation patterns, we will examine the Th2 locus throughout fetal and neonatal ontogeny for the presence of histone marks permissive and/or restrictive for gene expression. Lastly, additional expression arrays as well as whole genome methylation profiling will be performed to identify other likely regulators by specifically focusing on the time period during the transition from the fetal/neonatal hypomethylated to the adult hypermethylated state (fetal life though 9 days post birth). Overall, we will bring advanced molecular, cellular, and whole animal techniques to bear to understand how the distinct epigenetic profile at the Th2 locus is regulated throughout ontogeny. These studies will be particularly meaningful since (a) they rest on a platform of an extremely well characterized biological system and (b) they examine a locus, the Th2 locus, of high physiological relevance to immune function in early life.